The present invention relates to a three-dimensional measuring apparatus having an optical probe built therein for measuring the three-dimensional shape of a curved face such as an aspherical lens or the like in a non-contacting manner with superhigh accuracy.
The simplest three-dimensional measuring apparatus incorporating an optical probe is equipped with a microscope as a probe. The microscope is moved to a position where an image is monitored most clearly, and the x, y, and z coordinates at the position are read. The measuring apparatus of this type has the drawbacks, for example, that an image is not clearly found at a point of an error in focal depth or at a smooth surface without a flaw such as a lens surface. Even a measuring apparatus with an auto-focusing function based on an optical trigonometrical measurement cannot measure an inclined surface.
In a focus adjusting optical system disclosed in U.S. Pat. No. 2,897,722, the light is condensed at a surface to be measured and the reflecting light from the surface is divided into two by a beam splitter. The difference of the quantities of light passing through pin holes provided in front of and to the rear of (behind) the focal point is used as an error signal for focus adjustment. Accordingly, the prior system makes it possible to measure an inclined surface inclined in an optional direction. Nevertheless, errors occur with the prior art system, depending on the inclining direction of the surface.
That is, in the focus adjusting method of U.S. Pat. No. 2,897,722, since the direction of light passing through the beam splitter differs depending on the inclining direction of the surface to be measured, the separating or dividing ratio of light is changed according to the focus adjusting method disclosed in U.S. Pat. No. 2,897,722. This results in the focal point being different for different inclining directions of the surface to be measured, thus causing a measuring error.
Another prior art shape measuring apparatus, revealed in Japanese Laid-open Patent Publication No. 2-134506 by one of the inventors of the present invention, utilizes a focus controlling method which is capable of controlling an optical probe supported by a coil spring with considerably high accuracy, i.e., within approximately 0.01 .mu.m to the focus in about 20 mm range in the z-axis direction by means of a linear motor.
However, the following disadvantages are inherent in the structure of the shape measuring apparatus of Japanese Laid-open Patent Publication No. 2-134506, wherein a linear motor for driving the optical probe is arranged above an optical probe and a coil spring is arranged above the linear motor:
(1) It is desirable that the driving position of the linear motor is in the vicinity of the center of the z-axis movable section. Otherwise, during driving of the linear motor the torque for rotating the movable section worsens the moving perpendicularity in the z-axis direction. If the moving perpendicularity in the z-axis direction is poor, the measuring accuracy is deteriorated and the focus controlling efficiency is lowered. Although it is better to support the supporting spring at the center of gravity thereof, the prior art structure cannot realize this.
(2) It is difficult to extend the movable range in the z-axis direction of the optical probe so as to obtain a larger measuring area, because the space is too limited to provide a magnetic circuit of a length not smaller than the movable range of the linear motor.
Moreover, in the above shape measuring apparatus, the optical probe of the shape measuring apparatus is supported by the coil spring, and the tension of the coil spring is changed due to the expansion/contraction thereof in accordance with the movement of the optical probe, thus bringing about errors in focus servo. Moreover, if the movable range of the optical probe in the z-axis direction is increased, it causes the optical probe to become unable to track the focal point.